Regular foundry sands are minerals dug from the ground or crushed from rock. Typical examples include silica sand, olivine sand, zircon sand and chromite sand. Silica sand accounts for approximately 90% of the sands used in the foundry industry. The other three sands are more thermally stable, but more expensive--zircon being the most thermally stable and most expensive. Neither of these sands is porous and neither contains any volatile matter.
Sand molds shape the outside of castings. Cores are sand shapes which are positioned inside the mold to shape the inside of a casting. If a core were not used, the casting would be solid metal and many castings are not solid, but have inside channels or configurations.
Molds are one of two kinds:
(1) "green" sand molds are bentonite (clay)/water bonded sand mixtures rammed against a pattern to form a desired contour (a top half or cope and a bottom half or drag are booked together to form a complete mold cavity). The sand is a tough, pliable mixture which will hold its molded shape. Molten metal is poured into the mold cavity where it solidifies to form the resultant casting.
(2) "rigid" molds are sand mixtures which can be molded against a pattern and then hardened into a rigid condition. The method of hardening depends on the kind of binder used. Although bentonite bonded molds can be hardened by air-drying or baking, usually rigid molds are bonded with organic resins which harden into much stronger and harder shapes. Binders are designed to be hardened by several methods. Some are baked; some are cured or hardened by chemical reaction with a reagent; and some are hardened by flushing with a reactive gas.
Cores are usually rigid shapes employing the same kinds of binders and methods described above for rigid molds.
Much as pavement buckles on a hot day, a sand mold or core can buckle due to expansion during the casting operation. The high temperature expansion buckle of the mold wall causes a defect on the casting surface known as a "buckle" or a "scab". If a core expands too much, the core will crack or craze and metal will enter the crack to form an irregular fin of metal on the cored surface of the casting which must be removed. Obviously, less thermal expansion in a sand is a great advantage. U.S. Pat. Nos. 2,830,342 and 2,830,913, are directed to the excellent thermal stability of carbon sands.
Relatively inexpensive silica sand grains bound together with a suitable binder are used extensively as a mold and core material for receiving molten metal in the casting of metal parts. Olivine sand is much more expensive than silica sand but, having better thermal stability than silica sand, provides cast metal parts of higher quality, particularly having a more defect-free surface finish, requiring less manpower after casting to provide a consumer-acceptable surface finish. Olivine sand, therefore, has been used extensively as a mold and core surface in casting non-ferrous parts in particular and has replaced silica sand in many of the non-ferrous foundries in the United States.
Spherical or ovoid grain, carbon or coke particles, known to the trade as petroleum fluid coke, also have been used as foundry sands where silica sands and olivine sands do not have the physical properties entirely satisfactory for casting metals such as aluminum, copper, bronze, brass, iron and other metals and alloys. Such a fluid coke carbon sand presently is being sold by American Colloid Company of Arlington Heights, Ill. under the trademark CAST-RITE.RTM. and has been demonstrated to be superior to silica sand and olivine sand for foundry use.
Roasted carbon sand as described in U.S. Pat. No. 5,094,289, is a low cost carbon sand designed primarily for low melting temperature metals, such as aluminum and magnesium. Roasting at 1300.degree.-1400.degree. F. will remove all of the volatile matter which would otherwise be evolved if raw fluid coke were exposed to aluminum poured at 1400.degree. F. Likewise, thermal expansion would be minimal at 1400.degree. F. However, such relatively low temperature roasting does not eliminate porosity in such carbon sand.
Not until the work on the roasted carbon sand described in U.S. Pat. No. 5,094,289 was the full import of porosity in carbon sand realized. Previously, it was believed that raw fluid coke was only moderately porous. It was believed that the evolution of volatile matter, as gases during calcining, created the porosity and that once the porosity occurred, it remained.
Investigations leading to the present invention revealed that porosity exists in raw fluid coke grains and is increased slightly at roasting or calcining temperatures up to about 1900.degree. F. Then, particularly at about 2000.degree. F., the coke apparently shrinks sharply, closing the pores and eliminating the porosity. Increasing the calcining temperature above 2000.degree. F. does not necessarily shrink the coke further. However, in practice, a kiln operated at a considerably higher temperature, such as 2600.degree. F., for example, would likely heat the coke faster and would not allow a significant amount of time at about 2000.degree. F. (soaking time at 2000.degree. F.) to allow full shrinkage to occur. Further, calcining at 2600.degree. F. causes the evolution of volatile gases in a more explosive manner, thereby increasing the formations of pores. It is essential to the present invention that the rate of heating the coke from ambient temperature to about 2000.degree. F. be controlled to avoid the rapid evolution of volatile gases. Typically, a heating rate of about 25.degree. F. to 50 .degree. F. per minute has been satisfactory. Shock heating, i.e., instant exposure of room temperature coke to 2000.degree. F. furnace temperature will cause increased porosity.
Previously, carbon sands for foundry use have been produced by calcining fluid coke at various temperatures, none of which centered on a calcining temperature near 2000.degree. F., as disclosed herein.
U.S. Pat. Nos. 2,830,342 and 2,830,913 describe a carbon sand prepared by calcining fluid coke, specifying a "preferred method of calcination is to quickly heat the raw fluid coke up to about 2400.degree. F. to 2800.degree. F. . . . ." Porosity in the resultant product was acknowledged in the patents by the suggestion, ". . . to further pretreat it as by treatment with a solvent or by impregnating it with a suitable material such as water glass or finely divided graphite to decrease its porosity."
Under the protection of those patents, Humble Oil & Refining Company produced carbon sand (1961-1962) by calcining fluid coke at approximately 2500.degree. F. Porosity in that product was acknowledged in their sales literature by suggested remedies for liquid binder absorption.
Carbon sand was produced by Marathon Oil Company (1966-1967) by calcining fluid coke at approximately 2600.degree. F., however, the product was so extremely porous that the project was discontinued. Their unsolved problem with porosity is well documented.
Carbon sand was produced for Carbon Sands, Inc. (1985-1987) by calcining fluid coke at approximately 1850.degree. F. That product retained considerable porosity. (See Bakersfield Coke Table I, hereinafter.) Its applications as a foundry sand were restricted by the higher binder level required.
A carbon sand previously mentioned herein as a product (CAST-RITE 75) of American Colloid Company is being produced by calcining fluid coke at about 2200.degree. F. to about 2300.degree. F. but at a faster rate than disclosed herein. As shown in Table I, that carbon sand is somewhat porous and is inferior with respect to porosity to product prepared in accordance with the present invention, i.e., by calcining at 2000.degree. F.-2100.degree. F. (See Purvis Coke CAST-RITE 75 versus Purvis Coke Calcined at 2070.degree. F. in Table I.)
Since the calcining temperature in rotary kilns used to process fluid coke carbon sands is maintained by the burning of both the volatile hydrocarbon gases evolving from the coke and the carbon coke particles, a distinct advantage in yield and cost favors calcining at the lowest temperature that will produce good product. Therefore, the new technology of the present invention produces a better product and at a lower cost as well.
It is known that calcining at 2600.degree. F. produced carbon sand so extremely porous that cores made of it had almost no strength and hardness, when using normal amounts of liquid binder. Investigation for the present invention revealed that up to about 4.5% by weight water can be absorbed into porous carbon sand while having the visual appearance of dry sand. It follows that in a "green" sand molding mixture containing bentonite and water, an additional 4.5% by weight water would be needed to plasticize the bentonite since 4.5% by weight water is absorbed by the sand grains. Typically, green sand mixtures contain less than 4.5% water, therefore, porous green sand mixtures would necessarily contain twice as much (or more) water. Excessive water creates steam during pouring of molten metal causing casting defects. Therefore, the water content should always be held as low as possible in good foundry practice. Such absorptive porosity could not be tolerated in green sand molding mixtures.
Porous carbon sand will absorb some liquid binders used in cured molds and cores. To achieve adequate strength and hardness, up to twice as much binder may be required. The additional binder would generate additional decomposition gasses during pouring of the metal. Gas evolution from organic binders in cores and molds is a critical factor and a constant problem in foundries. A common casting defect known as a "blow" occurs when volatile gas cannot vent through the sand quickly enough, creating enough gas pressure to bubble through the molten metal, which may solidify before the gas escapes. The entrapped gas remains as an internal cavity in the casting, often times not revealed until the casting is purchased and machined by the customer. Thus, it must be appreciated that a method of preventing porous carbon sand is a breakthrough in carbon sand technology.
It should be recognized that the various commonly-used liquid binder systems vary greatly with respect to the amount and effect of absorption into porous carbon sand. More absorption occurs with thinner liquids, and with the longer time that the carbon sand/liquid binder mixture is held unused and uncured. Some two-part and three-part binder systems employ water-thin catalysts or reactants (such as phosphoric acid, and the like) which are readily absorbed.
In accordance with the following description of the present invention, the term "absorptive porosity" is used to refer to porosity in carbon sand. The following test procedure was used to measure absorptive porosity in accordance with the present invention.